Episodic pain is the most common acute morbidity and the leading cause of hospitalization in patients with sickle cell disease (SCD). It is caused by microvaso-occlusion induced by sickled red blood cells (RBCs), an outcome of the polymerization of deoxygenated hemoglobin S (HbS). Factors that contribute to the acute sickle pain include the release of cell-free DNA (cfDNA) and heme that initiate a downstream of events involving inflammation and thrombosis, and ischemia-reperfusion injury. cfDNA has been shown to be present in plasma of healthy subjects, but elevated in diseases and conditions that are characterized by increased cell death through necrosis or apoptosis. Indeed, we have previously shown that cfDNA in patients with SCD increased dramatically during acute painful episodes. During acute sickle pain, marked elevation of plasma hemoglobin has also been observed due to the acute increase in sickled RBCs and hemolysis. Both cfDNA and heme (break down product of hemoglobin) act as damage-associated molecular pattern (DAMP) molecules, initiating endothelial inflammation, stimulation of neutrophil extracellular trap (NET) formation, leukocyte recruitment, and microvascular thrombosis. Although there have been several studies of cytokines and chemokines in steady state and acute SCD, there has been no comprehensive study of how the inflammatory markers correlate with quantitative levels and profile of cfDNA. In this study, we would like to apply next generation sequencing (NGS) to analyze cfDNA from the plasma of patients with SCD in steady-state, and during painful crises to derive insights on the origin of tissue damage. In parallel with the free plasma DNA, we propose to measure markers of hemolysis and inflammation (cytokines, chemokines), and to investigate if interactions between these circulating molecules and blood cells (e.g. neutrophils) have the potential to modulate the progress and severity of the disease. In addition, we would also like to explore if there is a distinctive cell-free DNA and inflammatory signature in SCD in steady-state and during acute pain. Overall, this study provides an opportunity to evaluate new biomarkers of sickle cell pain crisis and to predict disease severity and prognosis. These measures may allow us to better understand the role of vaso-occlusion, hemolysis, and inflammation-related events and responses and serve as clinical endpoints in future studies of disease pathogenesis and/or therapeutic intervention for sickle cell disease. The protocol is actively accruing patients from the NIH clinical center. The first patient was enrolled on March 17, 2017. To date, 69 patients have been enrolled with 34 patients completing all phases of the study, the target fro completion of all 3 phases is 40. Total enrollment target is 60 subjects. 40 age and sex matched controls have been enrolled. Total enrollment target for controls is 40 subjects.